Suspect bad RAM as cause for freezing iMac screen

Intermittently my 2020 27" iMac will become unresponsive. The screen becomes a faint gray color. The cursor is frozen. The computer is completely unresponsive and requires a reboot. So far this has never happened when I am sitting at the iMac. I have the screen saver set to come on after 2 minutes of inactivity and the display to turn off after 10 minutes.

I have run Apple's hardware test several times and it shows no problems. I have run Techtool pro memory test option about 10 times. Eight out of ten times it reports no problems. The other two times it reports failure at "Extended March C-" portion of test.

I have run Rember app over a dozen times. About half of the time it reports no issues. The other half of the time I get a FAILURE message. The log refers to "data mismatch at local address......" Here is the full log report on one such test:

Memtest version 4.22 (64-bit)

Licensed under the GNU General Public License version 2 only

macOS 12.4 (21F79) running in multiuser mode

Memory Page Size: 4096

System has 20 Intel core(s) with SSE

Requested memory: 17721MB (18582421504 bytes)

Available memory: 17721MB (18582421504 bytes)

Allocated memory: 17721MB (18582421504 bytes) at local address 0x000000010f7db000

Attempting memory lock... WARNING: Testing with unlocked memory may be slower and less reliable

ERROR: Memory lock failed - reason unknown.

Partitioning memory into 2 comparison buffers...

Buffer A: 8860MB (9291210752 bytes) starts at local address 0x000000010f7db000

Buffer B: 8860MB (9291210752 bytes) starts at local address 0x00000003394a5000

Running 6 test sequences... (CTRL-C to quit)

Test sequence 1 of 6:

Running tests on full 17721MB region...

Stuck Address : setting 1 of 16

FAILURE! Data mismatch at local address 0x000000049865e788

Actual Data: 0x0000000c9865e788

Linear PRN : setting 1 of 16

Does this conclusively identify the problem as a bad RAM component? If so, how do I determine which of my 4 modules is causing the problem?

I will greatly appreciate any help you can offer.

John

Posted on Jun 12, 2022 5:20 PM

Top-ranking reply

HWTech

Level 9

68,696 points

Posted on Jun 13, 2022 11:18 AM

The only way to figure out which module may be bad is to remove one of them and test again. Rinse & repeat until the test runs without an error. I would start with the third party memory as it is much more likely to be the source of the problem than the original Apple memory. Keep in mind the problem may be with a specific memory slot, or even a specific combination of memory modules. It is all trial and error.

I like to use Memtest86 which can be booted from a USB stick. It would be a much better option than testing memory while booted into macOS where macOS & third party apps may interfere and limit what can be tested. If TechTool provides a bootable USB stick option for testing the memory, then that is also an option. Unfortunately you will need to run the memory tests extensively after you believe you have located the bad RAM module since the issue is so intermittent. The older v7.5 version of Memtest86 allows the tests to loop more than four times unlike the newer versions which restrict the free version to just four test loops at a time.

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Suspect bad RAM as cause for freezing iMac screen

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